This invention relates to a polymer derived from the reaction of hydrophilic monomeric components. More specifically, it relates to such a polymer incorporating an acyclic monomeric component especially adapted for use in the preparation of ophthalmic lenses, particularly soft hydrogel contact lenses.
The fabrication of thin hydrogel contact lenses with high water contents is one of the driving forces behind new developments in the ophthalmic art. Lenses with high water contents provide the cornea of the wearer with the oxygen it needs to maintain normal metabolism. Thus, a high degree of patient comfort can be obtained even though the lens may remain intact on the cornea for extended periods of time.
Unfortunately, high water content hydrogels are generally characterized by poor mechanical properties. This means that thin contact lenses made from these high water content hydrogels have poor mechanical properties, and therefore are very difficult to handle. It has therefore been desirable to develop high water content hydrogels, which have good mechanical properties. Unfortunately, these two properties are often inconsistent with one another.
One known method for improving the mechanical toughness of hydrogel lenses is to add monomers with hydrophobic character to the polymerization reaction mixture from which the lens is derived. Such monomers include lauryl methacrylate. Hydrophobic monomers tend to reduce the equilibrium water content, and thus the oxygen permeability of hydrogel polymers. This in turn causes an associated increase in patient discomfort and corneal swelling when the lens is placed on the cornea for extended periods of time.
The water content of the lens can be increased by the addition of highly hydrophilic monomers; however, there are two drawbacks associated with this remedy. The use of anionic monomers such as methacrylic acid results in hydrogels with high affinities for certain proteins, e.g. lysozyme. Alternatively, the addition of non-ionic hydrophilic monomers such as N,N-dimethylacrylamide (DMA) or N-vinylpyrrolidinone (NVP) can cause the resulting monomer blend to dissolve polystyrene. This factor is significant because it is often desirable to use polystyrene molds to form lenses.
Accordingly, interesting attempts have been made to develop hydrogels with high water content, good mechanical properties, and acceptable compatibility with polystyrene molds. For example, U.S. Pat. No. 5,006,622 discloses soft hydrogel contact lenses prepared from the reaction product of a branched alkylhydroxy cycloalkyl acrylate, and a hydrophilic monomer, such as hydroxyethyl methacrylate (HEMA). Further studies of these hydrogel systems can be found in J. Kunzler, G. Friends, R. Ozark and J. McGee, Polym. Mater. Sci. Eng. 1991, 239-240; and G. Friends and J. Kunzler, Polym. Mater. Sci. Eng. 1990, pp. 809-813. Although the water contents and physical properties of these lenses are reported to be promising, the complexity of steps necessary to synthesize the cycloalkyl acrylate is too unduly burdensome to be of any value.
Another attempt is disclosed in U.S. Pat. No. 3,983,083. This patent discloses a soft contact lens composed of an hydroxyalkyl methacrylate and another hydrophilic monomer. However, this lens composition fails to achieve the overall balance of properties desired for straightforward manufacture and acceptable wear characteristics.
In view of the deficiencies of the prior art, it would be beneficial to develop a polymer system for the manufacture of ophthalmic lenses, especially hydrogel contact lenses. More specifically, it would be especially advantageous to prepare soft hydrogel contact lenses from a hydrogel polymer system characterized by a high water content, good mechanical properties, and acceptable compatibility of the monomeric components with polystyrene.